This invention relates to marine seismic sources and, more particularly, to such sources in which a slug of liquid is propelled into surrounding water by an expanding volume of pressurized gas
In seismic surveying in a body of water, air guns submerged in the water are a common means for generating seismic energy to investigate geological conditions and formations in the earth below or adjacent to the body of water. For this purpose, one or more such air guns are submerged in the water; compressed air, or other gas or gases under pressure, is fed to the submerged guns and temporarily stored therein. At the desired instant, the air guns (the seismic energy sources) are actuated, i.e., fired, and pressurized gas, usually highly compressed air, is abruptly released into the surrounding water. In this manner powerful acoustical waves are generated. These waves are capable of penetrating deeply into the subsurface material of the earth to be reflected and refracted therein by the various strata and formations. The reflected or refracted acoustical waves are sensed and recorded to provide information and data about the geological conditions and formations.
In order to avoid the generation of strong secondary impulses, such as can be created by the oscillating bubble of discharged air from an individual air gun, seismic energy sources known as "water guns" have recently been used. One such water gun is discussed in an article by P. Newman in the Oil and Gas Journal, Aug. 7, 1978, Pages 138-150. In that device, water is pushed from the gun by a piston propelled by expanding pressurized gas. After such a water gun is fired, the expanded pressurized gas is vented into the surrounding water over a relatively long period of time such that the discharged air does not generate any significant seismic signal which would interfere with the seismic signal generated by the water slug. The water gun is then reset, and it is recharged by refilling the gun with pressurized air.
Another type of water gun is described in U.S. Pat. No. 4,234,052 of the inventor herein. In the gun disclosed and claimed therein, pressurized gas is retained, and the gun is recharged by a high pressure water pump.
Insofar as their operation is concerned, the basic difference between air guns and water guns resides in the type of acoustic "signature" which each generates. The initial blast of an air gun is followed by an ascending bubble of air which periodically expands and contracts as it rises to the surface. This results in a rather complex "signature" distribution. The water gun, on the other hand, ejects a sudden slug of relatively incompressible water which generates a cavity which then implodes. This implosion results in a much briefer report with fewer and simpler waveforms. Accordingly, it will be understood that some investigators prefer water guns over air guns and may prefer one or the other for specific applications. It should also be understood that for marine seismic surveying, the air gun is most common, and these are found in great numbers. A single array may include, for example, 20 to 50 such guns towed by a survey vessel. It thus will be understood that the substitution of water guns with their attendant firing and control system for air guns on a ship could be expected to involve very large expenses and considerable delays and inconveniences, because an entirely new system is being substituted and installed on shipboard.
Accordingly, it is a primary object of the present invention to provide method and apparatus for converting an air gun to a hydro gun conveniently and reliably, while enabling the air gun to be fired normally by triggering with a solenoid valve in the usual manner and while providing for automatic returning of the water-ejecting piston to its initial position after each firing.
Advantageously, the conversion from air gun to hydro gun is convenient and is reversible from one to the other and then back again, if desired; so that a survey ship is enabled to convert quickly and easily back and forth between air gun action and hydro gun action. Tremendous economic savings and savings in time are achieved as compared with prior art water gun installations, by virtue of the fact that the usual firing and control systems are enabled to be retained on shipboard and by virtue of the fact that the existing expensive air compressor equipment is also retained.
Other objects, features, and advantages will become apparent from the following description and appended claims.